YELLOWSTONE NATIONAL PARK, Montana (CNN) -- The gray wolf was officially removed from the Endangered Species Act's "threatened" list Friday after three decades -- a decision that has stoked controversy among environmentalists and ranchers.

Federal officials estimate there are 1,500 gray wolves in Yellowstone National Park.

It means the wolves can be shot and killed once they step out of Yellowstone National Park as soon as the affected states establish a hunting season. However, state Fish and Wildlife officials can shoot the animals whenever they deem the wolves to be a problem.

The government delisted the wolves -- which were eliminated from Yellowstone decades ago before being reintroduced in the 1990s -- because they are now thriving in the park that is dominated by bison, elk and bighorn sheep.

"They're back here in the Northern Rockies; they're back here in Yellowstone," said Doug Smith, a biologist for the National Park Service in Yellowstone.

"We're not ready to pop the champagne corks and have a party," said Doug Honnold, the managing attorney for Earthjustice, a non-profit environmental law firm based in Oakland, California, that has threatened to sue the government.

"My biggest fear is we're going to go backwards instead of forwards."

It wouldn't be the first time. In the early 20th century, wolves were the targets of a massive government extermination campaign.

"It's harder to find an animal more persecuted than wolves. ... We did wolf extermination with a vengeance," said Smith.

But attitudes began to change in the 1980s. Elk and bison populations increased dramatically because there was no natural predator to keep their numbers in check. In 1995, Smith led a team to bring wolves back to the Rocky Mountain landscape. They transplanted dozens of wolves to Yellowstone from Canada. See photos of Yellowstone's animals »

The project has been regarded as an overwhelming success.

There are now more than 1,500 wolves across Wyoming, Idaho and Montana, according to the government. That number convinced federal wildlife officials to remove them from the endangered species list.

Federal officials require each of the three states to maintain a population of 100 wolves, meaning a total of 300 wolves across all three states. The states have actually pledged to keep the population higher than that, at a rate of 150 wolves per state.